Field of the Invention
The present invention relates to a plasma CVD apparatus and a vacuum treatment apparatus, and particularly to a plasma CVD apparatus configured to form a carbon protective film of a magnetic recording medium and a vacuum treatment apparatus using the plasma CVD apparatus.
Description of the Related Art
There are high needs for a higher-capacity recording device such as a magnetic hard disk drive (HDD), and therefore demands exist for an increase in the areal recording density of a magnetic recording medium. The recording density of a magnetic recording medium depends on factors such as the characteristics of a magnetic film of the magnetic recording medium (such as coercive force and film thickness), the characteristics of a magnetic head (such as frequency characteristics and gap length), and spacing between the magnetic film of the magnetic recording medium and the magnetic head (referred to below as the “magnetic spacing”). Hence, the recording density can be increased bye for example, decreasing the film thickness of a protective film on the magnetic recording medium to reduce the magnetic spacing or decreasing in the medium noise caused by reducing magnetization inversion of the magnetic recording medium.
Nowadays, a recording and reproducing technique employed by a hard disk drive is mainly perpendicular magnetic recording. A magnetic recording medium used in the perpendicular magnetic recording includes: a rigid, non-magnetic substrate ouch as a glass substrate or a substrate formed by aluminum-alloy plated with nickel-phosphorus; and on the non-magnetic substrate, a multi-layered film including an adhesion layer, a soft magnetic layer, an intermediate layer, and a magnetic recording layer. A diamond-like carbon (DLC) protective film is formed on the magnetic recording layer to in order to meet corrosion resistance and product durability. The film thickness of the DLC protective film has been reduced to 5 nm or less. A fluorocarbon-based lubricant is applied onto the DLC protective film to reduce the wear between the magnetic head and the magnetic recording medium.
In a known method, the DLC protective film is formed with chemical vapor deposition (CVD) using plasma of a hydrocarbon gas. Specifically, the DLC protective film is formed by performing CVD film formation in which negative voltage is applied to the substrate, on which the magnetic recording layer is formed, to attract positive ions of hydrocarbon to the substrate.
Japanese Patent Nos. 3299721 and 3547202 disclose hot-filament plasma CVD apparatuses. With the techniques disclosed in Japanese Patent Nos. 3299721 and 3547202, however, the filament easily breaks and has to be replaced every time the filament breaks, which is inconvenient. Various technical improvements have been attempted for an apparatus for forming a DLC protective film without using a filament. For example, Japanese Patent Application Publication No. 2005-23403 discloses a method for forming a DLC protective film by using a high-frequency plasma CVD method. The technique disclosed in Japanese Patent Application Publication No. 2005-23403, however, poses the following inconvenience. Specifically, When DLC protective films are formed successively, hard DLC protective films having nigh internal stress are deposited on a high-frequency voltage-applied electrode due to self-bias effect. Having high internal stress, the DLC protective films are liable to peel off and therefore require short-interval maintenance.
There are also known technical improvements attempted to prevent the peeling of the deposited DLC films. For example, Japanese Patent No. 4268234 discloses an oxygen plasma cleaning technique for periodically removing DLC film deposited in a chamber for forming a protective film. In addition, Japanese Patent No, 4567867 discloses the following technique. Specifically, in addition to the DLC film removal by oxygen plasma cleaning, the surface of a discharge electrode is made of carbon so that impurities other than carbon may not be incorporated into or may not contaminate the protective film. In these techniques, however, the recording and reproducing performance may be degraded because the oxygen gas used in the plasma cleaning contaminates she surface of the magnetic layer. This degradation in performance needs to be prevented.